Supermassive black holes in deep multiwavelength surveys

Abstract

In recent years deep X-ray and infrared surveys have provided an efficient way to find accreting supermassive black holes, otherwise known as active galactic nuclei (AGN), in the young universe. Such surveys can, unlike optical surveys, find AGN obscured by high column densities of gas and dust. In those cases, deep optical data show only the host galaxy, which can then be studied in greater detail than in unobscured AGN. Some years ago the hard spectrum of the Xray "background" suggested that most AGN were obscured. Now GOODS, MUSYC, COSMOS and other surveys have confirmed this picture and given important quantitative constraints on AGN demographics. Specifically, we show that most AGN are obscured at all redshifts and the amount of obscuration depends on both luminosity and redshift, at least out to redshift z ∼ 2, the epoch of substantial black holes and galaxy growth. Larger-area deep infrared and hard X-ray surveys will be needed to reach higher redshifts and to probe fully the co-evolution of galaxies and black holes. Cosmic Growth of Black Holes and GalaxiesAbundant evidence indicates that the growth of a supermassive black hole is closely tied to the formation and evolution of the surrounding galaxy. The energy released from accretion onto the black hole affects star formation in the galaxy, probably limiting growth at the high-and low-mass ends, and of course the distribution and angular momentum of matter in the galaxy governs the amount of matter accumulated by the black hole (Silk & Rees 1998;King 2005;Rovilos et al. 2007). Emergent energy from accretion is also a factor in understanding ionization and radiation backgrounds (Hasinger 2000;Lawrence 2001). Understanding the growth history of these black holes is therefore critical to understanding the global evolution of structure in the Universe.Yet the demographics of supermassive black holes remain elusive. The largest samples of quasars and Active Galactic Nuclei (AGN), by which we mean supermassive black holes in a high accretion-rate phase †, have been found through optical selection (e.g., the Sloan Digital Sky Survey quasar sample; Schneider et al. 2002Schneider et al. , 2007, but, at least locally, these are not representative of the larger AGN population. Instead we need surveys less biased against obscured AGN.There are three reasons to suspect that most AGN are obscured by large column densities of gas and dust. First, a large body of evidence suggests that local AGN have geometries that are not spherically symmetric, and that different aspect angles present markedly different observed characteristics; this is referred to as AGN unification (Antonucci 1993;Urry & Padovani 1995). Second, AGN are more common at high redshift (z ∼ 2 − 3), where the average star formation rate is higher and thus it is even more likely that gas and dust surround the galaxy nucleus than at z ∼ 0. Third, and most important, obscured AGN are required to explain the shape of the X-ray "background" radiation.The X-ray "background" is actually the superposition of ind...

Cosmic X-ray surveys of distant active galaxies

“…Over the past 15 yr, a number of other relevant in-depth reviews have been prepared that have some overlap with the topics discussed here. These include Hasinger and Zamorani (2000), Gilli (2004), Brandt and Hasinger (2005), Urry and Treister (2007), Hickox (2009), Brandt and Alexander (2010), , , Kormendy and Ho (2013), Merloni and Heinz (2013), Shankar (2013), and Gilfanov and Merloni (2014). We encourage interested readers to consult these reviews as well, noting that they generally emphasize somewhat different topics than those emphasized here.…”

Cosmic X-ray Surveys of Distant Active Galaxies: The Demographics, Physics, and Ecology of Growing Supermassive Black Holes

“…Over the past 15 yr, a number of other relevant in-depth reviews have been prepared that have some overlap with the topics discussed here. These include Hasinger and Zamorani (2000), Gilli (2004), Brandt and Hasinger (2005), Urry and Treister (2007), Hickox (2009), Brandt and Alexander (2010), Alexander and Hickox (2012), , Kormendy and Ho (2013), Merloni and Heinz (2013), Shankar (2013), and Gilfanov and Merloni (2014). We encourage interested readers to consult these reviews as well, noting that they generally emphasize somewhat different topics than those emphasized here.…”